Daze fasteners

ABSTRACT

A daze fastener system for connecting two or more structural elements wherein the structural elements and fastener parts have substantially different coefficient of thermal expansion physical property characteristics is employed in this invention. By providing frusto-conical abutting surfaces between the structural elements and fastener parts any differences in thermal expansion/contraction between the parts is translated to sliding motion and avoids deleterious thermal stresses in the connection. An essential feature for isotropic homogeneous material connections is that at least two sets of mating surfaces are required wherein each set of mating surfaces have line element extensions that pass through a common point.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION

In the development of aerospace hardware it is often necessary tointerconnect two or more structural panels or elements while permittinglimited relative movement caused by thermal expansion or contraction ofthe parts during extreme temperature cycling as encountered in spaceflight. Thermal expansion or contraction of any arbitrary point in ahomogeneous material at a uniform temperature relative to any referencepoint is directly proportional to the radial distance between thearbitrary point and the reference point. Restraint of the thermalexpansion or contraction by a second homogeneous material surroundingthe first homogeneous material will produce thermal stresses in bothmaterials. This thermal stress condition will exist when the extensionof line elements of all mating or contacting surfaces between the twomaterials do not pass through a common reference point and when the twomaterials have different coefficients of thermal expansion (when thetemperatures of both materials is changed from some initial temperature)and when the materials each possess a modulus of elasticity.

If the coefficient of thermal expansion of the first homogeneousmaterial is greater than that of the surrounding material, thermalstress will be generated when the materials are heated. Also, if thecoefficient of thermal expansion of the first homogenous material isless than the surrounding material, thermal stresses will be generatedwhen the materials are cooled.

With a surrounding material initially contacting the homogenous materialby mating surfaces (whose line element extensions do not pass through acommon reference point when extended), separation of the materials willoccur as the temperature is changed from the initial temperatureprovided: (1) when the coefficient of thermal expansion of thehomogenous material is greater than that of the surrounding material,the materials will separate when cooled; (2) when the coefficient ofthermal expansion of the homogeneous material is less than thesurrounding material, the materials will separate when heated.

Maintaining contact between materials of different coefficient ofthermal expansion as the temperature fluctuates may be as significant tothe successful operation of structural devices utilizing these laws ofphysics as the avoidance of restraint of expansion.

To prevent either thermal stresses in, or separation of, the materialsit is essential that the extension of line elements of all matingsurfaces of the joined materials pass through a common reference point.The mating surfaces may be any combination of planes, cones andpyramids. Moreover, the line elements, when extended, of at least twosuch of all other mating surfaces must pass through the reference pointand when the extended line elements of all other surfaces do not passthrough the reference point these surfaces are required to be separatedto avoid thermal stress when the thermal expansion is such to causethese mating surfaces to interfere with one another.

U.S. Pat. No. 4,229,018 issued to Bickerstaff et al on Nov. 10, 1981,applies some of the principles of the present invention although thestructure and function of the respective devices are significantlydifferent. Bickerstaff et al disclose a roll with tires clamped to it byfrusto-conical rings of the same material composition as the roll shaftbut different from the tires. The entire roll assembly rotates as it andothers of like construction are pulled through and along the floor of anoven while transporting a sheet of glass resting thereon. The matchingfrusto-conical contact between the retaining rings and tires aredesigned so that differential thermal expansion between these partscauses sliding movement only of these frusto-conical surfaces relativeto each other.

It is therefore an object of the present invention to provide a fastenerthat extends through two or more structural element parts that are to beconnected in such a manner as to avoid thermal stresses in the connectedarea of the connected parts during thermal cycling.

It is a further object of the present invention to provide a fastenerfor alleviating thermal stresses in connected structural parts whereinthe fastener and structural parts have substantially differentcoefficients of thermal expansion.

A further object of this invention is a fastener extending through twoor more structural parts connected thereby wherein fastener/structuralpart contact is along mating planes.

A further object of the present invention is to provide a connectorelement for two or more structural parts that maintains contact betweenthe parts connected and the mating surfaces of the fastener duringthermal cycling.

An additional object of the present invention is a fastener forextending through and connecting two or more structural elements whereinthe fastener/structural element contact is along mating surfaces and anextension of the line elements of the mating surfaces pass through acommon point.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, the foregoing and additional objectsare attained by providing two or more structural panels to be joinedwith confronting and countersunk apertures extending completely orpartially therethrough and a fastener element positioned and securedwithin these apertures in fixed relationship therewith. The fasteneremployed in the preferred embodiment consists of an enlargedfrusto-conical head bolt adapted to snugly engage the countersunk areasof the two structural panels and having an elongated threaded extension.A first washer formed of the same material as the structural panels isdisposed over this threaded extension via an opening therein exceedingthe diameter of the threaded bolt extension so as to provide a spacingbetween the bolt and the interior diameter of the washer. A second metalwasher having essentially the same interior diameter as the exteriordiameter of the bolt is positioned over and against the first washer bya suitable threaded nut to thereby force the frusto-conical head of thefastener element into retained engagement with the two structural panelsbeing joined. A cotter pin is then positioned through a hole in the endof the threaded bolt extension to ensure retention of the nut thereon.The frusto-conical end of the fastener element is provided with asuitable opening therein for inserting a screw driver, hex wrench or thelike to assist in tightening the nut on the bolt. In lieu of thethreaded bolt arrangement, the frusto-conical head fastener element maybe in the form of a rivet and the elongated extension thereof deformedover the second washer. Also in lieu of the frusto-conical head, afrusto-conical insert of the same material as the fastener may beemployed and the fastener element may be provided with a central openingtherein to permit separate conventional bolt or rivet connectors beingemployed. Also, a double frusto-conical insert or head may be providedat opposite panel surfaces to form the connector element. In thisembodiment, each part has an elongated tubular end and an enlarged headshaped to snugly fit the countersunk exterior aperture of the structuralelements being joined and the tubular parts are disposed in telescopicalrelationship and internally positioned within the apertures. Suitablestructure such as a through extending bolt/nut or a rivet extendsthrough the fastener parts for retention of the fastener within theconnected parts. The telescoped tubular ends of the fastener are of asmaller outside diameter than the interior diameter of the structuralelement apertures so as to permit relative thermal expansion thereofduring temperature cycling of the connected parts. The enlarged head endof each fastener part mates with the countersunk aperture in the surfaceof one of the connected structural elements. These mating surfaces orplanes of the connector elements have line extensions which pass througha common single point. The line elements of mating surfaces can be anyshape surface without generating thermal stress while maintaining aconstant contact pressure provided the extension of the line elements ofall mating surfaces pass through the same point. As least two suchmating surfaces with coincident vertices are required and any othermating surface that does not have line elements that pass through thesame point of coincidence as these two must have a clearancetherebetween to avoid thermal stress, i.e., these other mating surfacescannot contact one another at any temperature if thermal stress is to beavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily apparent as the samebecomes better understood by reference to the following description whenconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a part sectional view of the preferred embodiment of thefastener system employing a bolt/nut connection according to the presentinvention;

FIG. 2 is a view similar to FIG. 1 illustrating a rivet connection;

FIG. 3 is a similar view illustrating an alternate embodiment of thefastener system of FIGS. 1 and 2;

FIG. 4 is a view similar to FIG. 3 of another connection illustrating abolt/nut connection;

FIG. 5 is a part sectional view of an alternate embodiment of thefastener of the present invention adapted for blind fastening;

FIG. 5a is an end of view of the FIG. 5 embodiment;

FIG. 6 is a part sectional view of an embodiment of the presentinvention connecting multiple panels together;

FIG. 7 is a sectional view of a fastener system according to the presentinvention connecting relatively thick non-isotropic panels; and

FIG. 8 is a view similar to FIG. 7 and illustrating a slightmodification thereof.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like elements are referred to bythe same reference numeral throughout the several views and moreparticularly to FIG. 1, there is shown the preferred embodiment of thedaze fastener system according to the present invention and generallydesignated by reference numeral 10. Fastener 10 in this embodiment isemployed to connect two structural panels 11 and 13 formed ofcarbon-carbon composite or other material useful for extreme high or lowtemperature structural applications and wherein the panel materialcomposition has a large difference in coefficient of thermal expansionfrom that of the fastener material. Fastener 10 must provide a snug fitbetween panels 11 and 13, at all temperatures during extreme temperaturecycling as encountered in aerospace structural applications, while alsoremaining free of thermal stress. As shown in FIG. 1, panels 11 and 13are provided with adjacent frusto-conical apertures 15, 17 extendingtherethrough and receive bolt 19. Bolt 19 is provided with afrusto-conical head 20 adapted to snugly fit within and extend throughapertures 15 and 17. A spacer washer 23 is positioned over shank 21 ofbolt 19 to abut against panel 13. Spacer washer 23 is provided with aninterior diameter opening 24 exceeding the exterior diameter of shank 21so as to provide a clearance therebetween and is designated by referencenumeral 25. Spacer washer 23 thickness is selected to provide adequatebearing area between the head 20, and panels 11 and 13 and is formed ofthe same material composition as these panels. A metal washer 27 of thesame material as bolt 19 and of substantially the same interior diameteras the exterior diameter of shank 21 is also disposed on shank 21 so asto engage spacer washer 23. A hex nut 29 of the same material as bolt 19is threaded onto shank 21 of bolt 19 to firmly engage metal washer 27and a conventional cotter pin 30 is placed through shank 21 and a groove31 provided in an extension on hex nut 29. As shown by dotted lines,line element extensions from the surfaces of conical head 20 extendthrough a common point P on a plane with metal washer 27 and the matingsurface of spacer washer 23.

Referring now more particularly to FIG. 2, a rivet 35 having afrusto-conical head 36 mating with frusto-conical apertures 15, 17 ofpanels 11 and 13, is provided with a malleable shank portion 37. Afterpositioning enlarged spacer washer 23 and metal washer 27 over shank 37,the end thereof is deformed in a conventional manner to provide the ovalterminus 39 thereon to engage and compress washer 27 against spacerwasher 23 and thereby fasten panels 11 and 13 together. As in FIG. 1,the dotted line element extensions of frusto-conical head 36 extendthrough a common point on the plane of metal washer 27 and the matingsurface of spacer washer 23.

Referring now to FIG. 3, a hollow daze insert fastener 40 is employed toconnect panels 11 and 13. Fastener 40 in this embodiment is essentiallyfunnel shaped with an enlarged frusto-conical end 41 and a reduceddiameter tubular end 42. In this embodiment, a rivet having afrusto-conical head 44 is disposed within the hollow cone area offastener insert 40 and the malleable shank portion 43 of the rivetextends through the elongated tubular end 42 of fastener 40. Afterpositioning spacer washer 23 and metal washer 27 over the tubular end 42of fastener 40, malleable shank 43 is deformed in a conventional mannerto provide an oval terminus 45 thereon to engage and compress washer 27against spacer washer 23 as in the preceding embodiment.

Referring now to FIG. 4 another hollow daze insert fastener is shown andgenerally designated by reference numeral 50. Fastener 50 is alsoessentially funnel shaped and has an enlarged frusto-conical end 51 anda reduced diameter tubular end 52. In this embodiment, a bolt 53 havinga head 54 and a threaded shank 55 is disposed through fastener insert 50with the head 54 maintained in the hollow frusto-conical end 51.Fastener 50 is utilized to connect panels 11 and 13 by tightening nut 29onto threaded bolt shank 55 after insert fastener 50, bolt 53, spacerwasher 23 and metal washer 27 are positioned as shown in FIG. 4. As inFIG. 1, a conventional cotter pin 30 is positioned through a suitableopening (not shown) in shank 55 to retain nut 29 in locked position.Head 54 of bolt 53 is also provided with a depressed area 56 thereinadapted to receive a suitable socket wrench, screw driver or the like toaid in tightening nut 29.

Referring now to FIGS. 5 and 5a, a daze insert fastener for blindattachments is illustrated and designated generally by reference numeral60. In this embodiment, insert fastener 60 is provided with an enlargedpyramid shaped head 61 and a reduced diameter tubular shank 62 extendingthrough a panel 65. A metal washer 63 is disposed in a depressed area 66provided in the opposite surface of panel 65 from the pyramid shapedaperture 64 therein receiving pyramid head 61 of insert 60. The plane ofthe mating surfaces of washer 63 and panel 65 passes through the commonvertices of insert frusto-pyramid head 61 and frusto-conical head 74 ofbolt 73. The tubular shank 62 is provided with either longitudinallydisposed multiple splits or flared to permit bending thereof 90°relative to the center axis of the tubular shank 62 to abut againstmetal washer 63 and retain fastener insert 60 in position. Two splitsegments of tubular shank 62 are shown in FIG. 5 and designated byreference numerals 68 and 69. The square end of pyramid head 61 is shownmore clearly in FIG. 5a and includes an integral central extensionthereon as designated by reference numeral 70. Extension 70 is providedwith internal threads and serves to receive the threaded shank 72 ofbolt 73 and the extension 70 is split wherein the metal between thesplits is deformed inward during manufacture of frusto-pyramid head 60to provide a self-locking feature for bolt 73. The opposite end of bolt73 is an enlarged frusto-conical head 74 received within afrusto-conical aperture 76 provided through panel 75. In operation,panel 65 is provided with pyramid shaped insert fastener 60 securedtherein prior to permanent attachment of panel 65 in operative positionby suitable mechanism (not shown). Subsequent assembly or fastening ofpanel 75 is then made possible by bolt 73 passing therethrough forconnection with threaded extension 70 provided on the blind side ofpanel 65. Tightening of bolt 73 is facilitated by the square pyramidshape of insert 60 retaining the insert in a non-rotative mode byturning bolt 73 by aid of a screw driver, torque socket wrench, or thelike, inserted in opening 77 in head 74 thereof.

Referring now to FIG. 6, multiple carbon-carbon or like homogeneousstructural panels may also be connected by use of the daze fasteners ofthe present invention without danger of thermal cycling stresses causingthe connections to fail. In the illustrated embodiment three panelsdesignated by reference numerals 80, 81 and 82 are shown connected by adaze fastener system generally designated by reference numeral 87. Inthis embodiment, a pair of essentially hollow frusto-conical inserts 89,90 are disposed within confronting frusto-conical apertures formedthrough respective panels 80, 81 and 82 of the panel group. The interiorpanel 81 is provided with an opening 83 therein disposed coaxial andintermediate to the confronting conical apertures. Inserts 89 and 90 arefastened together via bolt 92 as in the preceding embodiments. The shankportion 93 of bolt 92 disposed within interior panel 82 is by design ofa smaller exterior diameter than the coaxial opening in panel 81 toprovide adequate clearance between the bolt and panel to permitanticipated expansion/contraction of the parts during extremetemperature cycling.

In each of the embodiments of FIGS. 1-6 described hereinabove, thepanels connected are of homogeneous isotropic construction and thefastener elements employed are homogeneous materials that exhibitextreme different coefficients of thermal expansion (α). In each ofthese embodiments it is essential that line element extensions of two ormore of the mating surfaces of the fastener elements pass through acommon point. In the embodiments utilizing the metal washer connectorelement, this common point is also on a plane with the metal washerwhere it contacts the spacer washer.

Referring now more particularly to FIGS. 7 and 8, the application of thepresent invention to connect relatively thick carbon-carbon or likestructures is illustrated. As shown, the coefficient of thermalexpansion (α) of the materials 101 and 102 being joined and spacerwasher 106 therein is greater through the thickness than in plane.However, the fastener 104 and the metal washer 107 are made ofhomogeneous material. In these instances the line element extensions maydeviate from a common point by a distance (ε) according to the formula

    ε=(α.sub.cc)/α.sub.m)t

wherein

ε is the distance permitted for the vertex of line element extensions ofthe insert to deviate from the mating plane of the connecting metalwasher and the spacer washer;

α_(cc) is the coefficient of thermal expansion for the carbon-carbonpanels being connected;

α_(m) is the coefficient of thermal expansion of the washer and insertmaterial fasteners; and

t is thickness of the carbon-carbon spacer washer.

As shown in FIG. 7, non-isotropic carbon-carbon panels 101, 102 areprovided therein, confronting and frusto-conical apertures (notdesignated) are provided in panels 101 and 102 of such dimension as toreceive in contacting engagement therewith hollow frusto-conical head103 of fastener insert 104. Insert 104 is provided with an elongatedmalleable segment 105 extending through panels 101 and 102. Afterpositioning spacer washer 106 and metal washer 107 on segment 105, theend thereof is deformed to provide terminus 108 thereon in contact withmetal washer 107 to retain panels 101 and 102 connected by fastenerinsert 104. As in the previous embodiments the opening or interiordiameter of spacer washer 106 is larger than the exterior diameter ofsegment 105 to permit relative thermal expansion/contraction of theparts without inducing stress thereon.

Referring now to FIG. 8, a slight modification of that shown in FIG. 7is illustrated wherein a spot face surface area is removed from theinterior of panel 101 to provide an opening 110. This area serves topermit bending of panel 101 during thermal expansion/contraction in thisregion to further reduce thermal stresses and provide a snug fit betweenthe panels 101, 102 and between the panels and frusto-conical head 103.In each of FIGS. 7 and 8 the frusto-conical head is shown as hollow butthis is primarily for a weight saving measure and solid connectors maybe employed as desired. Also, bolt type connectors could be employed inlieu of the rivet connection in each of the embodiments of FIGS. 7 and 8if so desired.

The embodiments of FIGS. 1 to 6 may also be used for joiningnon-homogeneous materials that have a greater coefficient of thermalexpansion through their thickness than in plane by making the metallicfastener of a material (also non-homogeneous) that has a highercoefficient of thermal expansion along its length (L) than across itsdiameter (D). This was accomplished by adding fiber reinforcement withinthe fastener material. The amount of reinforcement is that required toproduce the same ratio of α (L/D) as through-the-thickness to in-planeof the elements being joined.

Although no specific materials have been designated for the panels andfasteners discussed in the preferred embodiments herein, the inventionis specifically designed for joining elements that have a largedifference in coefficient of thermal expansion from that of the fastenerelements. Specifically, the invention is useful in joining carbon-carboncomposites structural panels, ceramic panels, glass and the like withmetal and metal alloy fastener elements. The term carbon-carboncomposite as employed herein refers to any carbon fiber-carbon matrixstructure. In one specific embodiment panel 11, 13 and spacer washer 23were constructed of a carbon-carbon composite obtained from the VoughtCorporation and the fastener elements including the metal washer 27 wereconstructed of an oxide dispersion strengthened alloy (MA-956E) capableof withstanding thermal cycling from cryogenic up to 2200° F. MA-956E isa product of Huntington Alloy, Incorporated. The invention is notrestricted to these specific examples and obviously other materialshaving the physical property characteristics needed for specificapplications are obviously considered within the teachings of thisinvention.

The mode of operation of the invention highlights the novelty thereof.When a structural body joined by the daze fastener system of the presentinvention is subject to extreme elevated temperatures, the metalfastener components, which have a much greater coefficient of thermalexpansion than the members being joined expand in all directions fromthe coincident vertices of the cone formed by the frusto-conical headsof the rivet or bolt and the plane of the mating surface between themetal washer and the spacer washer. Consequently, the bearing surfacesbetween the head and the carbon-carbon panels remain undisturbed, i.e.,the frusto-conical head slides along the countersunk surfaces in thecarbon-carbon as the head expands, and the metal washer slides along thespacer washer as the metal washer and nut expand. The head neitherdeparts from nor expands into the carbon-carbon. The contact forcebetween the fastener and the carbon-carbon remains constant for alltemperatures. A radial clearance is provided around the cylindricalportion of the insert to allow unrestrained thermal expansion of thispart of the fastener. Since no part of the fastener is restrained fromexpanding as it is heated, no thermal stress is induced in either thefastener or the material being joined. Thus, for a large difference incoefficients of expansion (α) between the fastener and the materialbeing joined, zero thermally induced strain (ε) or thermal stress ispresent in either the fastener or the material being joined, because thesubject fastener is free to expand thermally without restraint by thecarbon-carbon, a feature provided by the novel structure of theinvention. The acronym "daze" has been coined to characterize thisfeature. However, another significant feature is that a snug fit existsbetween the parts being joined and between the fastener and the partsbeing joined at all temperatures. The amount of bearing force (snugnessof fit) may be controlled during manufacture of the structure either bythe length of the cylindrical portion of the insert (which may beshimmed) or by the flaring of the tubular end of the rivet. Differentcone angles may be used for the rivet head and insert and the cone anglecan be selected greater than 0° and less than 90°.

Although specific embodiments of the invention have been describedherein they are to be considered as exemplary of the novel featuresthereof and are not exhaustive. There are obviously many variations andmodifications of these specific exemplary examples that will be readilyapparent to those skilled in the art in the light of the above teachingswithout departing from the spirit or scope of the appended claims. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A fastener for connecting two or morestructural elements having mating frusto-conical apertures therethroughto receive the fastener and wherein the structural elements and fastenerhave substantially different coefficient of thermal expansion physicalproperty characteristics comprising:a first elongated fastener elementhaving an elongated body portion and an integral head portion formed onsaid elongated body portion; said elongated body portion beingpositioned within and extending through the structural elements to beconnected thereby and said head portion being provided with afrusto-conical exterior surface and being received in mating surfacecontacting engagement with the frusto-conical apertures formed in thestructural elements to be connected thereby and wherein the matingsurfaces consist of line elements that when extended form a vertex; aspacer washer element disposed about said elongated body portion and inengagement with one of said structural elements to be connected; a metalwasher also disposed about said elongated body portion and in engagementwith said spacer washer wherein the contacting surfaces of said metalwasher and said spacer washer are on a plane that passes through thevertex formed by line element extensions of the mating surfaces of saidfrusto-conical head and said frusto-conical apertures; and means firmlyengaging said metal washer to force said metal washer against saidspacer washer and retaining the structural elements fastened together.2. The fastener of claim 1 including said elongated body portion havingexternal threads along at least a segment of the length thereof and saidmeans firmly engaging said metal washer is a nut threaded onto saidelongated body portion to force said metal washer against said spacerwasher and retain the structural elements fastened together.
 3. Thefastener of claim 1 including said elongated body portion being formedof a malleable metal and said means firmly engaging said metal washercomprises a deformed terminus of said elongated body portion engagingsaid metal washer.
 4. The fastener of claim 1 wherein said spacer washeris formed of the same material composition as the two or more structuralelements and this composition is an isotropic homogeneous material, saidspacer washer having an interior diameter exceeding the exteriordiameter of said elongated body portion and said means firmly engagingsaid metal washer is a threaded nut, and said elongated body portionbeing provided with an exteriorly threaded segment to receive saidthreaded nut.
 5. The fastener of claim 1 wherein said head portion andsaid elongated body portion thereof are of a hollow substantiallyfrusto-conical funnel configuration and including a separate elementdisposed within and securing said funnel configuration to retain thestructural elements fastened.
 6. The fastener of claim 5 wherein saidseparate element is an elongated bolt having a head end disposed withinthe hollow funnel configuration area and an elongated threaded shankportion extending through said elongated body portion and receiving athreaded nut thereon serving as said means to firmly engage said metalwasher.
 7. The fastener of claim 5 wherein said separate element is anelongated rivet having a frusto-conical head received within the hollowfunnel configuration and a malleable shank received through said spacerwasher and said metal washer with a deformed terminus thereon serving assaid means to firmly engage said metal washer.